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BUG: KCSAN: data-race in generic_file_buffered_read / ondemand_readahead

write to 0xffff888096eb0fb0 of 8 bytes by task 18394 on cpu 1:
 generic_file_buffered_read+0x186c/0x1fa0 mm/filemap.c:2427
 generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2507
 ext4_file_read_iter+0x2d8/0x420 fs/ext4/file.c:74
 call_read_iter include/linux/fs.h:1876 [inline]
 generic_file_splice_read+0x22b/0x310 fs/splice.c:312
 do_splice_to fs/splice.c:890 [inline]
 splice_direct_to_actor+0x2a8/0x660 fs/splice.c:970
 do_splice_direct+0xf2/0x170 fs/splice.c:1079
 do_sendfile+0x56a/0xba0 fs/read_write.c:1548
 __do_sys_sendfile64 fs/read_write.c:1609 [inline]
 __se_sys_sendfile64 fs/read_write.c:1595 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1595
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888096eb0fb0 of 8 bytes by task 18398 on cpu 0:
 ondemand_readahead+0x501/0x6e0 mm/readahead.c:510
 page_cache_sync_readahead+0x178/0x1a0 mm/readahead.c:585
 generic_file_buffered_read+0x2de/0x1fa0 mm/filemap.c:2199
 generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2507
 ext4_file_read_iter+0x2d8/0x420 fs/ext4/file.c:74
 call_read_iter include/linux/fs.h:1876 [inline]
 generic_file_splice_read+0x22b/0x310 fs/splice.c:312
 do_splice_to fs/splice.c:890 [inline]
 splice_direct_to_actor+0x2a8/0x660 fs/splice.c:970
 do_splice_direct+0xf2/0x170 fs/splice.c:1079
 do_sendfile+0x56a/0xba0 fs/read_write.c:1548
 __do_sys_sendfile64 fs/read_write.c:1609 [inline]
 __se_sys_sendfile64 fs/read_write.c:1595 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1595
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 18398 Comm: syz-executor.1 Not tainted 5.9.0-rc8-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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